The present invention relates to a semiconductor substrate and a method of manufacturing the semiconductor substrate, and more specifically to a SOI (Silicon On Insulator) substrate and a method of manufacturing the SOI substrate.
Conventionally, there have been so far known such a semiconductor substrate that a thin monocrystal silicon film is formed on a silicon substrate via an insulating film, which is referred to as a SOI substrate. The SOI substrate has various features as follows:
Where an ordinary silicon substrate (not the SOI) is used, there exists such a tendency that short channel effect as to a MOS transistor is easily produced with the advance of the microminiaturization and higher integration of a semiconductor device. As one of the methods of preventing the occurrence of the short channel effect of the MOS transistor, it is effective to increase the dopant impurity concentration at a device forming region on the substrate surface. However, in the conventional technique for introducing dopant impurities into an ordinary silicon substrate in accordance with ion implantation, where a microminiaturized device whose gate length is as small as 0.25 .mu.m or less is required to be formed, since the dopant impurity concentration cannot be controlled sufficiently, it is difficult to control the threshold voltage of the MOS transistor. In contrast with this, in the case of the SOI substrate, since a perfect depletion can be realized at the channel portion of the MOS transistor, it is possible to easily prevent the short channel effect of the MOS transistor.
Further, in the case of the SOI substrate, since a parasitic capacitance inevitably produced at the source-drain diffusion layer can be reduced, it is possible to increase the operational speed of the MOS transistor.
In addition, when the SOI substrate is used, since there exists such a possibility that the device structure and the device manufacturing process can be both simplified, the semiconductor technique dependent upon the SOI substrate has been noticed more and more recently from this point of view.
As a first method of manufacturing the SOI substrate as described above, there exists a method as follows: two monocrystal silicon substrates are bonded to each other. In more detail, an insulating film (e.g., a silicon oxide film) is formed on one surface of one of the monocrystal silicon substrates, and the other of the monocrystal silicon substrates is bonded to this formed insulating film. Further, an outer surface (a surface not the bonded surface) of one of the two monocrystal silicon substrates is polished down to a thin film. After that, the polished surface is further dry-etched for planarization.
Further, as a second method of manufacturing the SOI substrate, there exists a method as follows: after an insulating film has been formed on a monocrystal silicon substrate, an amorphous silicon layer is further formed on the formed insulating film. Further, the formed amorphous silicon layer is once melted with a laser, for instance so that another monocrystal silicon layer can be formed by re-crystallization.
In addition, as a third method of manufacturing the SOI substrate, there exists a method called Separation by Implanted Oxygen (SIMOX) as follows: after oxygen ions have been implanted onto a monocrystal silicon substrate at a high concentration, a buried oxide film is formed by thermal oxidation to obtain both an insulating layer and a monocrystal silicon layer.
However, the SOI: substrate manufactured in accordance with the above-mentioned conventional methods as described above involve the following drawbacks:
(1) When the above-mentioned SOI substrate is used, it is preferable to add a step of gettering contaminative impurities during the process of manufacturing the device. Accordingly, various methods have been so far proposed as follows: a first method is called Intrinsic Gettering (IG) in which oxygen precipitate is formed inside a monocrystal silicon substrate to reduce the metallic contaminant at a device forming region by allowing the formed oxygen precipitate to trap metallic contaminant); and a second method is called Backside Poly Sealing (BSP) in which a poly silicon layer is formed on a reverse surface of the substrate to reduce the metallic contaminant at an element forming region by allowing the formed poly silicon layer to trap metallic contaminant.
In the above-mentioned methods, however, since the gettering step is additionally required, there exits problems in that the number of manufacturing steps increases and thereby the manufacturing cost inevitably rises.
Further, in the case of the above-mentioned IG method and the BSP method, since the gettering performance is largely dependent upon the crystal defect density (Bulk Micro Defect density) induced by oxygen in the substrate and further the film thickness and the crystal characteristics of the polysilicon film formed on the reverse surface of the substrate, there exists such a problem in that a stable gettering performance can not be always obtained.
(2) Further, in the case of the method of bonding two monocrystal silicon substrates, a thermal treatment is necessary to bond two monocrystal silicon substrates to each other. In this heat treatment process, however, since crystal defect (such as oxygen precipitate) is often formed on the surface of the substrate, defective devices are often manufactured. Further, in the case of the method of bonding two monocrystal silicon substrates to each other, If the oxygen concentration in the thin monocrystal silicon layer is high, since thermal donors are produced from the oxygen in the thin monocrystal silicon layer during the bonding process of the two monocrystal silicon substrates, the defective devices are manufactured due to fluctuations in resistivity or due to crystal defect.
On the other hand, in order to eliminate the drawbacks as described above, it may be considered to previously heat-treat the monocrystal silicon substrate (on which the device is formed) under non-oxidation condition prior to the bonding process. When the non-oxidation heat treatment is effected, although the crystal defect can be eliminated almost perfectly, however, since the dopent impurities are also diffused outward due to the non-oxidation heat treatment, there arises another problem in that the dopant impurity concentration in the thin monocrystal silicon film is reduced.
(3) On the other hand, even in the case of the methods of forming a monocrystal silicon layer on the basis of the re-crystallization after malting with a laser and the SIMOX method, since it Is difficult to eliminate the defect in the thin monocrystal silicon layer, the practical SOI substrate cannot be so far obtained.